User-output devices such as displays or near-eye displays or three-dimensional audio systems may be used to provide outputs which are perceptible to a user. Near-eye displays are apparatus which enable an image to be provided proximate to the eye of the user. A user output device such as a near-eye display may be used in applications such as virtual reality or augmented reality applications. Apparatus such as near eye displays may be configured so as to enable overlaid viewing of virtual content and the real world.
It is useful to be able to control the images displayed on such displays or the outputs provided by such user output devices so that they are synchronised with movement of the user's head.